Pentamethyldisilane is a better hydrogen donor than Et 3SiH toward tert-butoxyl (k's for the overall reaction are 17 and 5.7 × 10 6 M -1 s -1, respectively, at ca. 27°C) and toward primary alkyl radicals (k's for Si-H bond cleavage are ca. 10 and 1.0 × 10 4 M -1 s -1, respectively, at 120°C). Absolute rate constants for various reactions of the Me 3SiSiMe 2·radical have been measured by laser flash photolysis at room temperature. In halogen atom abstractions, for example, the Me 3SiSiMe 2·radical is slightly less reactive than Et 3Si·, but it is more reactive than n-Bu 3Ge· and n-Bu 3Sn·. Pentamethyldisilane would appear to offer an attractive alternative to n-Bu 3SnH and n-Bu 3GeH in radical chain reactions in which the desired product is formed by a hydrogen transfer not to the organic radical formed initially but to a second radical formed by a slow β-scission or rearrangement of the initial radical.